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XYLENE POWER LTD.

FNR REACTIVITY

By Charles Rhodes, P.Eng., Ph.D.

This web page is concerned with the change in FNR reactivity with temperature which is the mechanism normally used for FNR liquid sodium temperature control.
 

FNR REACTIVITY TRIMMING:
The neutron gain per fission step in an operating FNR can be expressed in the form:
Gf X Fc X Exp(- Vn Sigmaf Nf - Vn SigmaU Nu - Vn Sigmas Ns - Vn Sigmai Ni - Vn Sigmac Nc) ~ 1
where:
Gf = 3.1 neutrons out / neutron in = ideal fission neutron gain for Pu-239
Fc ~ (1 / 2) = fraction of neutrons produced that remain in the core rather than diffusing into the blanket
Vn = fast neutron velocity
Sigmaf = Pu-239 fast fission cross section
Nf = average Pu-239 atom concentration
Sigmau = U-238 fast neuton absorption cross section Nu = average U-238 atom concentration
Sigmas = sodium atom fast neutron absorption cross section
Ns = average sodium atom concentration Sigmai = iron fast neutron absorption cross section
Ni = average iron atom concentration
Sigmac = chromium atom fast neutron absorption cross section
Nc = average chromium atom concentration

During the working life of a fuel bundle operated to 15% burnup the Pu fraction drops from 20% to about 12.7%. Thus Nf varies from its initial value of Nfo to its final value of 0.635 Nfo. To maintain a reactivity of ~ 1.0 it is necessary to compensate for the change in Nf by increasing Fc which is adjusted by further insertion of mobile fuel bundles. Typically when the fuel is new Fc ~ 0.5 and when the fuel is ready for reprocessing Fc ~ 0.9. Most of the nuclear heat is injected into the middle core zone. A complication with this strategy is that the aging of each fixed fuel bundle is determined by the aging of the various adjacent mobile fuel bundles. Thus keeping track of the aging of the various fuel bundles is a complicated process.

Determine the fraction of fission neutrons absorbed by sodium both in the reactor core and blanket and via neutron leakage into the guard band.

This web page partially updated March 17, 2020.

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